It's really most likely mites. If one rat has it, it's likely all your animals have it. I've dealt with mites in the past, and it's very easy to treat (had to treat our rabbits too at the same time as the rats). Even if all the rats don't have scabs, the location of the scabs definitely sounds like mites and its better to be safe than sorry and not let the affected rat be miserable while getting eaten. Get Revolution (other flea meds can be deadly, make sure its revolution/selamectin) get the kitten one and give 1-2 drops on their back directly on the skin where they hopefully can't groom it off. Make sure the fur dries before putting them back in the cage. Treat EVERYONE or they will just reinfect each other (even if the others don't have scabs yet from scratching themselves, they will still have mites if living with the scabby one. He is probably just more bothered by them than the others). You can freeze leftover Revolution and treat again in 2 weeks to make sure the mites are wiped out for good. This will also take care of lice in case that's what it is too.